Grüneboom et al. Nat Metab https://doi.org/10.1038/s42255-018-0016-5 (2019)

Blood exchange between the general circulatory system and the interior of long bones is quick, but the means of entry and exit has been elusive. It turns out that there were hidden structures just waiting to be revealed.

To look beneath the long bones, researchers from University Duisburg-Essen in Germany used a technique called simpleCLEAR to make murine femurs and tibiae transparent and then imaged the samples with light-sheet fluorescence and X-ray microscopes. In addition to 16 nutrient arteries and a central sinus with two exits to supply blood flow, they discovered hundreds of previously unknown capillaries that line the entire lengths of the bones and connect their inner and outer surfaces. Those small capillaries have big impact—over 80% of arterial and 59% of venous blood circulate through them.